1. Field of the Invention
The present invention relates to a projection type display device constructed with a reflection type liquid crystal display device, more particularly to a projection type display device capable of enhancing contrast of display images and preventing a quality deterioration of the display images due to disclination.
2. Description of the Related Art
In recent years, a projection type display device using a liquid crystal display device is proposed. This projection type display device is often used in the form of an image magnification equipment at presentations or in a home theater. Thus, a variety of displaying formalities have been developed.
Especially, as discussed in Japanese Patent Publication Laid-open No. 2007-101764, the projection type display device using a reflection type liquid crystal display device is capable of displaying bright images at a high resolution. Because the reflection type liquid crystal device comprises one substrate having transparent electrodes formed on its surface, another substrate opposed to the former substrate and having reflecting electrodes and drive circuits for every pixel arranged in matrix and a liquid crystal layer enclosed between these opposing substrates, allowing the drive circuits for liquid crystal to be arranged underside the pixels for display.
In addition, since the liquid crystal layer utilizes the reflection type liquid crystal display device of vertically-aligned liquid crystals, and the projection type display device is adapted so as to allow light to be incident on the reflection type liquid crystal display device through a wire grid polarizing beam splitter (referred to as “WG-PBS” hereinafter), the same device is capable of display images at high contrast.
Such a projection type display device is shown in FIG. 1. The projection type display device comprises a reflection type liquid crystal display (reflective LCD) device 1 and a wire grid polarizing beam splitter (WG-PBS) 2 arranged at an inclination angle of 45° to the reflective LCD device 1. The reflective LCD device 1 comprises a transparent substrate 11 having transparent electrodes formed thereon, an active matrix substrate 12 opposed to the substrate 11 and having reflecting electrodes and drive circuits for every pixel arranged in matrix and a liquid crystal layer 13 enclosed in a gap between the substrate 11 and the substrate 12.
The WG-PBS 2 includes a number of wire grids (WG) 2a formed on a transparent flat plate. In the WG-PBS 2, the wire grids (WG) 2a are arranged in parallel to the reflective LCD device 1. In illumination light incident on the WG-PBS 2, its P polarization component as the incident light on the LCD device 1 is transmitted through the WG-PBS 2, while its S polarization component is reflected by the WG-PBS 2. The P polarization component of the illumination light transmitted through the WG-PBS 2 is then incident on the LCD device 1.
The reflective LCD device 1 polarizes and modulates the incident illumination light in accordance with image signals and then reflects the same light. The modulated light reflected by the LCD device 1 returns to the WG-PBS 2. In the modulated light, its S polarization component only is reflected by the WG-PBS 2, while its P polarization component is transmitted through the WG-PBS 2 to be a return light for the incident direction of the illumination light. Then, the modulated light reflected by the WG-PBS 2 is transmitted through an analyzer (e.g. a polarizing plate) 3 and then incident on a not-shown imaging lens system to project the light on a screen for imaging. Note that the analyzer 3 is an optical device for analyzing the modulated light reflected by the WG-PBS 2.
In the reflective LCD device 1, alignment layers 14, 15 are formed on respective surfaces of the transparent substrate 11 and the active matrix substrate 12 on the side of the liquid crystal layer 13, respectively. In operation, these alignment layers 14, 15 impart liquid-crystal molecules alignment condition to liquid crystal molecules forming the liquid crystal layer 13, as shown in FIG. 2. Japanese Patent Publication Laid-open No. 2007-101764 exhibits the liquid-crystal molecules alignment condition to obtain high-contrast display images in the projection type display device combining the reflective LCD device 1 having the liquid crystal layer 13a composed of such liquid crystal molecules in vertical alignment with the WG-PBS 2.
In this projection type display device, that is, the liquid-crystal molecules alignment in the liquid crystal layer 13 of the reflective LCD device 1 is defined as a direction to meet a predetermined condition in the relationship with the WG-PBS 2, as shown in FIG. 3 or FIG. 4. In the projection type display device, nematic LC (liquid crystal) having negative dielectric anisotropy is used as liquid crystal, and a pre-tilt angle θp is given to liquid crystal molecules 16-1, 16-2. Assume here, one of directions of projection lines obtained by projecting the wire grids 2a of the WG-PBS 2 on the active matrix substrate 12 vertically, the one direction being directed to a direction where one extension surface of the polarization spectroscopic surface intersects with another extension surface of the active matrix substrate 12 when the one direction is turned in the counterclockwise direction by 90°, is represented by X-axis. Based on the assumption, an angle α from X-axis up to a line segment 17-1 obtained by projecting respective a long axis of the liquid crystal molecule 16-1 onto the active matrix substrate 12 is 45° (see FIG. 3). Here, the line segment 17-1 is positioned in the counterclockwise direction from X-axis in view from the transparent substrate 11. Similarly, another angle α from X-axis up to another line segment 17-2 obtained by projecting respective a long axis of the liquid crystal molecule 16-21 onto the active matrix substrate 12 is also 45° (see FIG. 4). Here, the line segment 17-2 is positioned in the clockwise direction from X-axis in view from the transparent substrate 11.
As for the above liquid-crystal molecules alignment condition, if representing the projective line segments 17-1, 17-2 by vectors upon the establishment of X-Y-Z axes on the active matrix substrate 12 as shown in FIGS. 3 and 4, then respective vectors 17-1, 17-2 are illustrated on X-Y plane, as shown FIGS. 5A and 5B. When defining the azimuth angles of the liquid crystal molecules 16-1, 16-2 by angles φ from X-axis, the vector 17-1 is represented by an angle φ of 225° in the counterclockwise direction as shown in FIG. 5A, while the vector 17-2 is represented by an angle φ of 315° in the counterclockwise direction as shown in FIG. 5B.
In the reflective LCD device, meanwhile, it is generally believed when an angle between the direction of a line segment, which is obtained by projecting a liquid crystal molecule in the liquid crystal layer on a substrate surface, and the vibrating direction of an incident polarized light becomes near 45°, there can be attained a light state and a maximum output (i.e. brightness). In detail, as disclosed in the specification of U.S. Pat. No. 4,127,322, an output T in the liquid crystal display device can be represented byT=K·sin2(2φ)·sin2(π·Δneff·d/λ)  (1)here K is a constant, φ an azimuth angle, Δneff an effective double refraction of liquid crystal molecule, d a thickness of liquid crystal cell and λ is a wavelength of incident polarization light.
From the operation expression, it will be understood that the azimuth angles providing a maximum output are 45°, 135°, 225° and 315°.
That is, the maximum outputs in light state can be provided at four azimuth angles φ (i.e. 45°, 135°, 225° and 315° in the counterclockwise direction in FIGS. 5A and 5B).
On the other hand, the brightness in dark state at the azimuth angle φ of 45° in the counterclockwise direction or at 135° in the same direction greatly differs from the brightness in light state at the azimuth angle φ of 225° in the counterclockwise direction or at 315° in the same direction. Thus, in case of the azimuth angle φ of 225° or 315° in the counterclockwise direction, the brightness in dark state turns down. Therefore, under the liquid-crystal molecules alignment condition where the azimuth angle φ is either 225° or 315° in the counterclockwise direction, it becomes possible to realize the maximum contrast ratio while ensuring the output (brightness) in light state greatly.
While, aiming to provide a liquid crystal device using vertically-aligned liquid crystal molecules attaining high-contrast images in spite of obliquely-incident light, Japanese Patent Publication Laid-open No. 2002-072217 proposes a vertically-aligned reflection type liquid crystal display device including a pair of substrates having electrodes on their opposed substrate surfaces, a liquid crystal layer containing vertically-aligned liquid crystal molecules and interposed between the substrates, and alignment layers arranged on the substrate surfaces and alignment-processed so as to twist the pre-tilt direction of the vertically-aligned liquid crystal molecules between the substrate.
Further, in Japanese Patent Publication Laid-open No. 2007-212997, the twist angle is set to a predetermined condition and the application to a projection type display device using a WG-PBS is discussed.
However, Japanese Patent Publication Laid-open No. 2007-212997 does not teach a condition of realizing high-contrast images in the projection type display device combining a WG-PBS clearly.
In addition, although an application of slight slant (pre-tilt angle) on the liquid crystal molecules unidirectionally prevents the falling direction of the molecules from being disrupting to cause a disclination, it could not be eliminated perfectly. Thus, if applying this on the projection type display device combining a WG-PBS, there is a possibility of exerting a bad influence on the screen depending on the application. Typically, this bad influence would appear in the form of a phenomenon that an oblique line of a projected picture is displayed with a color far from the original color.